1. Field of the Invention
This invention generally relates to electronics and, more particularly, to battery power management in a wireless communications device.
2. Description of the Related Art
Portable devices, such as wireless communications devices, are equipped with batteries to provide power for device operations when the device is used in its portable mode. However, there are at least two problems associated with the use of batteries in a portable device. First, battery parameters and the voltage requirements for portable device circuitry can prevent a portable device from extracting the full amount of power available in a battery, as explained below. Second, because portable device batteries have limited capacities for storing and supplying power, batteries must be periodically recharged by an external power supply or battery charger. Unfortunately, the charger typically supplies voltage at a level higher than is usable by the battery.
FIG. 5 is a graph of portable device battery voltage level plotted against portable device battery power discharge (prior art). The axes of the graph are unitless. Regarding the first problem, FIG. 5 shows two voltage levels, Vckt and Vbatt. Vckt is the minimum voltage level required for the operation of the circuitry in a portable device and Vbatt is the minimum voltage level to which the battery can be discharged without harm to the battery. One curve shown on FIG. 5 is for a conventional battery, such as lithium ion, and the second curve is for a newer type of battery, lithium-sulfide (LiS). Note that the position and proportion of Vckt, Vbatt, and the battery curves are approximate and represent typical cases. Other positions and proportions are possible. When a conventional battery voltage level is above Vckt, the battery is able to supply power to operate the portable device. When the battery voltage level is between Vckt and Vbatt, the battery still has power available for use, but the battery is not providing voltage at a level sufficient to allow operation of the device. That is, the battery can still be discharged until the battery voltage drops to Vbatt. Lithium sulfide batteries have greater power storage capacity than lithium ion batteries, but as shown on FIG. 5, the voltage provided by the batteries can be below Vckt. Therefore, to extract all the power available in a conventional battery and to use the power available from a LiS battery, it is known to use a boost converter to raise the level of the voltage provided by a battery to a value at or above Vckt.
Regarding the second problem, the voltage incompatibility between an external power supply and a portable device battery, it is known to use a buck converter in the portable device to buck (step-down) the voltage level supplied by the external power supply to the device battery.
It is known to provide separate boost and buck converters in portable devices. However, there are at least two disadvantages associated with the use of separate converters: an increase in the number of components used in a portable device; and, a subsequent increase in the cost of the portable device. A wireless communications device is used as an example to illustrate these disadvantages. However, it should be appreciated that these disadvantages apply to other portable devices and even non-portable devices as well. Advances in technology and manufacturing techniques permit wireless communications devices to be made increasingly smaller, which in turn, creates a need to reduce the space required for components in the devices by reducing the number and/or size of the components.
Unfortunately, providing separate converters increases the number of components in a wireless device. The subsequent increase in space usage can limit the size to which a wireless device can be reduced, or can limit space available in the wireless device for other components. In most wireless devices, a buck converter may be necessary to properly charge the device battery. However, due to the space constraints noted above, a choice may be necessary between optimizing battery performance and optimizing space usage in the device. For example, a choice may be necessary between the options of adding a boost converter to maximize power utilization for a lithium ion device battery and omitting the boost converter to save space in the device. With a LiS battery, the problem is exacerbated. As described above, it may not be possible to use a LiS battery at all without a converter to boost the battery voltage to a level at or above Vckt.
Accordingly, it would be advantageous if the number of components used to buck and boost voltage levels in a portable device could be reduced. This would help reduce the size and cost of devices requiring buck and boost converters, particularly wireless communications devices.
The present invention addresses buck and boost power conversion operations in electronic devices, particularly wireless communications devices. The invention recognizes that space and costs for components are limitations in wireless communications devices. The invention addresses these problems by using a same set of components to perform both buck and boost power conversion operations in a portable device, thereby reducing the number of components used in the portable device.
Accordingly, a system is presented for bi-directional power conversion in a portable device. The system uses a single inductor to both buck voltage from an external power supply to a battery and boost battery voltage to a portable device power bus. A controller supplies control signals opening and closing first and second switches, enabling electric current paths for implementing buck and boost power conversion operations. The battery has a terminal connected to ground and a second terminal connected to an inductor terminal. The first switch has a terminal connected to the power bus, a second terminal connected to a second inductor terminal, and an input to accept a control signal. The second switch has a terminal connected to the second inductor terminal, a second terminal connected to ground, and an input to accept a control signal. The controller has inputs to accept the bus voltage and the battery voltage and outputs to supply power conversion control signals to the switches in response to evaluating the bus and battery voltages.
The controller supplies buck power conversion control signals to: close the first switch and open the second switch to build up voltage across the inductor to reduce the voltage supplied to the battery by an external power supply connected to the bus; and to open the first switch and close the second switch to discharge the inductor built-up voltage to the battery. The controller supplies boost power conversion control signals to open the first switch and close the second switch to build up voltage across the inductor and to close the first switch and open the second switch to add the inductor built-up voltage to the voltage supplied by the battery to the bus. In one aspect of the system, transistors, specifically metal-oxide-semiconductor field effect transistors (MOSFETs), are used for the switches.
Additional details of the above-described system, and a method for bi-directional power conversion in a portable device are provided below.